1. Field of the Invention
The invention relates to a method and an apparatus of measuring the temperature parameters of an ISFET, and more particularly, to a method and an apparatus of measuring the temperature parameters of an ISFET that uses hydrogenated amorphous silicon as a sensing film.
2. Description of the Prior Art
Ion sensitive field effect transistor (ISFET) is constructed by substituting a sensing film for the metal gate on the gate oxide of a traditional MOSFET. When the ISFET is dipped into a solution, the interfacial potential between the sensing film and the solution will influence the semiconductor surface since only an extremely thin dielectric (that is, the gate oxide) separates the sensing film and the semiconductor surface. This influences the charge density in the inverse layer of the semiconductor surface, and thereby modulates the channel current passing through the ISFET. Therefore, by utilizing this characteristic, the pH value or other ion concentration in the solution can be deduced from the measurement of source/drain current and the gate voltage of the ISFET. Furthermore, the potential difference on the interface between the sensing film and the solution is in relation to the ion activity in the solution. The hydrogen ion activity in the solution can be measured by using different channel currents caused by different interfacial potential differences in varied solutions with different hydrogen ion activity.
The patents, which are related to the formation of the ISFET or the measurement of the ISFET, are listed hereinafter.
(1)U.S. Pat. No.5,319,266 discloses a method of using Ta2O5 as the sensing film in the pH-ISFET. A radio frequency sputtering method is employed to sputter a Ta2O5 film of a thickness 400xcx9c500xc3x85 on a gate dielectric layer made of silicon nitride/silicon oxide. Then, an annealing process is performed in oxygen for one hour at a temperature controlled between 375 xc2x0 C. and 400xc2x0 C. Compared with the traditional pH-ISFET, this prevention has better sensitivity and reliability.
(2)U.S. Pat. No. 5,130,265 discloses a method of fabricating the ISFET with multiple functions. The method comprises using siloxanic prepolymer as the sensitive film, mixing the solution, photochemistry treatment and heat treatment.
(3)U.S. Pat. No. 5,035,791 discloses a method that uses an organic film such as polysiloxanic to form the sensing film in the ISFET and related processes of the device.
(4) U.S. Pat. No. 5,061,976 discloses a method that covers a carbon thin film on the gate oxide of the ISFET and then covers a 2,6 xylenol electrolytic polymerization film on thereof. The ISFET has the ability to sense hydrogen ions and the advantages of small drift time, high reliability and insensitive to light. When other thin films are covered on the ISFET, other kinds of ions can be sensed.
(5) U.S. Pat. No. 4,812,220 discloses a method of precisely measuring glutamate concentration by diminished enzyme sensor, wherein a method of fixing enzyme on the substrate and a converter that transforms the variations of matter amount into electric signals.
(6) U.S. Pat. No. 5,511,428 discloses an ISFET with a back contact structure that is applied to micro sensors such as a pressure sensor, a shearing stress sensor, and a temperature sensor or micro regulators.
(7) U.S. Pat. No. 4,691,167 discloses a method of measuring ion activity in the solution by combining the ISFET, the reference electrode, the temperature sensor, amplified circuits, calculation and memory circuits. Since the sensitivity is a function of temperature and drain current and is decided by a variable of gate voltage, the sensitivity can be obtained by calculating formulas stored in the memory.
(8) U.S. Pat. No. 4,660,063 discloses a method of performing both laser drilling and solid diffusion to form a three-D diode array on the semiconductor wafer. The laser is first employed to drill the wafer, and then the impurities are diffused from the hole to form a cylindrical PN junction and complete a non-plane ISFET structure.
(9) U.S. Pat. No. 5,911,873 provides a device that comprises ISFET, reference electrode device, ISFET control circuits, memory, measuring circuits and diagnosis circuits to measure ion concentration in the solution. The ISFET control circuits operate the equipment at a certainly drain/source voltage and a gate/source voltage that is relative to n continuous drain currents. The memory stores the repetitions of the ISFET characteristics, continuous drain current, and gate/source voltage. The measuring circuits measure ion concentration by using a group of n continuous drain currents and gate/source voltage and the repetition of the ISFET characteristics. The diagnosis circuits measure the ISFET characteristics by using n continuous drain currents and gate/source voltage.
(10) U.S. Pat. No. 5,309,085 integrates the measurinng circuit of a creature sensor having ISFET on a wafer. The measurinng circuit has two ISFET devices that are an enzyme ISFET and a reference electrode FET and whose output signals can be amplified by using a differential amplifier.
(11) U.S. Pat. No. 4,735,702 provides a polymer covered on the oxide layer of the ISFET, wherein a chemical bonding will be formed on the interface between the polymer and oxide layer so as to form the sensitive film.
However, since the ISFET is a semiconductor device, the change of temperature affects all data obtained by measuring the ISFET for a long time, and thus causes a measuring error. In order to prevent the measuring error, the ISFET must be operated under a specific temperature to ensure the accuracy of the measuring result. In view of the experimental conditions and facilities at the present day, controlling the measuring temperature to keep the ISFET in a constant temperature state is not a perfect method.
It is therefore a primary objective of the invention to provide a method of measuring the temperature parameters of an ISFET that uses hydrogenated amorphous silicon as a sensing film, which uses the measurements of the temperature parameters and the source/drain current and gate voltage in an unknown solution to deduce the ion concentration and the pH value of the unknown solution (such as sensitivity or temperature coefficient).
Another object of the invention is to provide an apparatus for measuring the temperature parameters of an ISFET that uses hydrogenated amorphous silicon as a sensing film, which uses ordinary functions of controlling temperature, measuring temperature and measuring current/voltage to completely measure the temperature parameters (such as sensitivity or temperature coefficient).
In order to achieve one object of the invention, a method of measuring the temperature parameters of an ISFET that uses hydrogenated amorphous silicon as a sensing film is provided. At the step a1, make the sensing film contact a buffer solution. At step a2, change the pH value of the buffer solution at a predetermined temperature and use a current/voltage measuring device to measure and record the source/drain current and the gate voltage of the ISFET so as to obtain a curve. At step a3, select a fixed current from the curve to obtain the temperature parameter at the predetermined temperature. At step a4, change the temperature of the buffer solution to repeat the steps a1 to a3 so as to obtain the temperature parameters at the different temperatures.
In order to achieve another object of the present invention, another method of measuring the temperature parameters of an ISFET that uses hydrogenated amorphous silicon as a sensing film is provided. At step b1, make the sensing film contact a buffer solution. At step b2, change the pH value of the buffer solution and use a current/voltage measuring device to measure and record the source/drain current and the gate voltage of the ISFET for a temperature range so as to obtain a curve. At step b3, use the curve to gain a zero temperature coefficient point and an average current at each pH value. At step b4, operate the ISFET at the average current and use the current/voltage measuring device to measure the gate voltage of the ISFET.
In order to achieve another object of the invention, an apparatus for measuring the temperature parameters of an ISFET that uses hydrogenated amorphous silicon as a sensing film is provided. The apparatus comprises a semiconductor substrate where the ISFET is formed wherein the semiconductor substrate comprises a pair of source and drain separated by each other and the sensing film insulated from the surface of the semiconductor substrate; a buffer solution for contacting the ISFET; a light-isolating container for loading the buffer solution and isolating light; a heater for heating the buffer solution; a temperature controller connected with the heater; a test fixture connected with the source and drain of the ISFET; and a current/voltage measuring device connected the test fixture.
It is an advantage of the invention that after measuring the source/drain current and the gate voltage in an unknown solution, the temperature parameters can be utilized to deduce the ion concentration or pH value of the unknown solution with unknown concentration.